Complex objects which may contain a variety of narrow apertures, holes or tubes are difficult to sterilize. In particular, open ended lumens, internal cavities, deadlegs and flat surfaces in close proximity present difficulties. In situ sterilization of freeze dryers and sterilization of deadlegs and lumens created by piping external to the freeze drying chamber that is corroded, has a small external leak, or an extremely high depth to diameter ratio can also present an extreme challenge. Moreover, lumens and deadlegs which absorb sterilant material to any degree can also be difficult to sterilize.
Sterilization of complex objects is currently accomplished by using wet or dry heat, chemicals, ionizing radiation, electron beams, microwaves, arc discharges, lasers, plasmas and high vapor pressure chemical gases. Heat, penetrating radiation, or high vapor pressure chemical gases, have been preferred for sterilizing articles of irregular shape because of their ability to effectuate sterilization within narrow apertures, holes and tubes which are otherwise difficult to access. Each of these methods, however, has limitations and problems.
For the purposes of this invention the term sterilization means a 6 log (or greater) reduction in bioburden.
A number of these sterilization methods are discussed in "Principles and Methods of Sterilization in Health Sciences", second edition, written by John J. Perkins and published by Charles C. Thomas of Springfield, Ill.
A table for dry heat sterilization containing adequate exposure times for a variety of temperatures contained in Perkins on page 289 is reproduced as Table A below.
TABLE A ______________________________________ Dry Heat Sterilization Time-Temperature Ratios Exposure Temperature Degrees C. Degree F. Exposure Time ______________________________________ 180 356 30 minutes 170 340 1 hour 160 320 2 hours 150 300 21/2 hours 140 285 3 hours 121 250 6 hours ______________________________________
Dry heat sterilization does not require any pressure, but it is very difficult, and quite impractical, to heat complicated objects such as an entire freeze dryer and its associated piping to these high temperatures using electric or gas heaters or with hot air.
Moist heat sterilization is much easier to implement since the introduction of saturated steam into a complicated object such as a freeze dryer will supply both the heat and the moisture. A table for moist heat sterilization containing adequate exposure times for a variety of temperatures (Perkins, page 161) is reproduced as Table B, below.
TABLE B ______________________________________ Moist Heat Sterilization Time-Temperature Ratios Exposure Temperature Corresponding Degrees C. Degree F. Pressure Exposure Time ______________________________________ 138 280 49.2 psia 0.8 Minutes 132 270 41.9 psia 2 Minutes 125 257 33.7 psia 8 Minutes 121 250 29.8 psia 12 Minutes 118 245 27.3 psia 18 Minutes 116 240 25.0 psia 30 Minutes ______________________________________
Both Tables A and B contain exposure times and do not account for the time required for all of the components within the object such as a freeze dryer and its associated piping to come up to temperature.
According to "Temperature Profiles and Sterilization within a Dead-ended Tube", written by Jack J. Young and Barbara L. Ferko and published in the July-August issue of the Journal of Parenteral Science & Technology, the time for a dead leg to come up to temperature can be considerable.
The data from Table III in Young et al for dead leg sterilization at 121.degree. C. is reproduced in Table C, below. Note that all of these times, which account for coming up to temperature, are much longer than the exposure times recommended by Perkins (Table B). Freeze drying piping dead legs are typically sloped at around 5.degree. so they will drain, and they often are longer than those discussed in Young, et al. Thus, it would be expected to require sterilization times in excess of 358 minutes to completely sterilize a freeze dryer and its associated piping.
TABLE C ______________________________________ Estimated Sterilization Times Within Dead-ended Tubes For Varying Tube Orientations Distance Percent Sterilization Time (minutes) up Tube into Tube Vertical up 45.degree. Up 5.degree. Up ______________________________________ 1.8 cm 19.2 29.8 24.0 23.5 3.1 cm 33.0 31.2 54.3 72.5 4.3 cm 45.8 64.4 117.3 206.0 5.6 cm 59.8 68.0 121.4 358.3 6.9 cm 73.6 101.3 N.T. N.T. 8.1 cm 86.4 167.3 N.T. N.T. ______________________________________
The time required to heat up, sterilize, and cool down a massive object such as a freeze dryer will substantially reduce the time available for the Object (freeze dryer) to be used for its intended purpose (freeze drying). The addition of "jackets" to heat and cool the chamber and condenser on a freeze dryer can decrease this time substantially, i.e. from 24 hours to 8 hours, but at the expense of thermally stressing the chamber, condenser and associated piping. This thermal stress, when alternated with the extreme cold (-40.degree. C.) associated with freeze drying will propagate leaks and can actually cause the chamber and/or condenser to crack and have to be replaced periodically at great expense in time.
Gaseous chemical sterilization agents such as ethylene oxide can sterilize within 21/2 hours, but an extended aeration time, up to 24 hours, is required to remove the residuals. Disposal of the expended sterilant is also difficult because it is considered both toxic and carcinogenic. Some states, California for example, require that any products that have been in contact with ethylene oxide been in contact with ethylene oxide be labeled as being processed with a known carcinogen. This would put a manufacturer at a disadvantage with a competitor who used a different sterilization process.
Use of pure concentrated ethylene oxide sterilant can be dangerous because it is explosive when mixed with oxygen (both during and at the end of the cycle when air is admitted into the chamber) so it is typically mixed with a diluent such as Freon (which is being banned because it is an ozone depleter) before it is introduced into the sterilization chamber.
Ionizing radiation must be of sufficiently high energy to penetrate articles effectively. This necessitates the use of x-rays and/or gamma rays, both of which require large and expensive apparatus and are generally hazardous. Furthermore, ionizing radiation could not be expected to penetrate effectively around through and into all of the metal components and down the piping within a complex object such as a freeze dryer.
Use of low vapor pressure chemical vapor sterilants avoid some of the above-mentioned concern and limitations, but because it is also difficult for them to penetrate into the holes, openings and apertures of complex shaped articles, several methods attempting to enhance their penetration characteristics have been considered. These methods typically include: (1) deep evacuation of the sterilizing chamber prior to introduction of the sterilant; (2) alternating of evacuation pulses and sterilant introduction pulses; (3) increasing sterilant concentration and/or pre-injection chamber pressure; (4) direct coupling and flowing or recirculating the sterilant through the lumen or object; and (5) continuously "pressure pulsing" during the sterilization phase.
U.S. Pat. No. 4,348,357 provides a method for plasma pulsations. U.S. Pat. No. 4,296,067 provides a method of sterilizing material, especially bandage and surgical instruments, in a steam autoclave operating as near to vacuum as possible. And finally, U.S. Pat. No. 4,372,916 discloses a method which utilizes alternating evacuation and sterilant introduction pulses.
Each of the above mentioned methods are designed to enhance sterilant penetration, but all continue to fall short of being ideal.
Achieving an increase in sterilant penetration performance by use of a deep vacuum as suggested by U.S. Pat. No. 4,296,067 has been verified. Tests ran by AMSCO, and contained in Table D, below verified that this method would work for hydrogen peroxide vapor. However, as seen in the table this concept when used with low vapor pressure gases requires the vacuum level to be of the order of 1 Torr or less to achieve best results. This requirement results in excessive pump down times, and expensive pumping equipment. In addition the results obtained using this technique are achievable with fewer deep vacuum pulses when using the invention proposed herein.
TABLE D ______________________________________ Average Hydrogen Peroxide Vapor Sterilant Penetration into 1 cm ID .times. 120 cm Deep Passivated Stainless Steel Deadlegs 21/2 Ft.sup.3 Chamber 154 Ft.sup.3 Chamber Pre-Injenction Depth of Penetration Depth of Penetration Vacuum Level (cm) (percent) (cm) (percent) ______________________________________ 10 Torr N.T. N.T. 60 50 5 Torr 80 67 60 50 2 Torr 80 67 73 61 1 Torr 90 75 87 73 0.1 Torr 115 96 N.A. N.A. ______________________________________
Sterilant penetration results were not available for the large chamber because the vacuum system was unable to evacuate to 0.1 Torr. A very expensive pump would have been capable of doing so but the cycle time would have increased substantially in the process.
A dead leg shape containing coupons inoculated with 1.times.10.sup.6 Bacillus Steorothemophilus spores as illustrated in Figure L was placed inside an 81 liter chamber that had a leak rate (pressure rise) of 150 microns per minute. The chamber was then evacuated to 0.1 Torr prior to the introduction of hydrogen peroxide vapor which increased the pressure to about 6 Torr. After a six minute sterilize hold the chamber was re-evacuated and the sterilize pulse repeated. After 9 sterilize pulses all the coupons were sterile.
This same dead leg was located external to, but attached to the chamber using the KF40 adapter. The chamber leak rate (pressure rise) now increased to 230 microns per minute due to a leak rate into the dead leg of about 0.0085 standard liters per minute. After a 9 pulse sterilize cycle, which was identical to that ran when the dead leg was inside the chamber, none of the coupons was found to be sterile. The enhanced penetration due to the use of a deep pre-injection vacuum was insufficient to overcome the small leak in the external dead leg.
Thus, when sterilizing large, complex objects such as a freeze dryer the deep pre-injection vacuum was found to be very expensive to implement, to have long cycle times and to be unable to sterilize external piping dead legs with small leaks.
The method of alternating evacuation pulses and sterilant introduction pulses discussed in U.S. Pat. No. 4,372,916 was evaluated on the 154 Ft.sup.3 chamber using hydrogen peroxide vapor. The results of this evaluation for an evacuation of 1 Torr are included in Table E. The test was conducted using lem I.D..times.120 cm deep passivated stainless steel dead legs containing inoculated with 1.0.times.10.sup.6 Bacillus steorothemophilus spores as the biological challenge.
TABLE E ______________________________________ Depth of Penetration Number Positive/Number Tested for Sterility from open 4 Sterilize 8 Sterilize 16 Sterilize 32 Sterilize end (cm) Pulses Pulses Pulses Pulses ______________________________________ 0 0/6 0/2 0/2 0/2 10 0/6 0/2 0/2 0/2 20 0/6 0/2 0/2 0/2 30 0/6 0/2 0/2 0/2 40 0/6 0/2 0/2 0/2 50 0/6 0/2 0/2 0/2 60 0/6 0/2 0/2 0/2 65 0/6 0/2 0/2 0/2 70 0/6 0/2 0/2 0/2 75 3/6 0/2 0/2 0/2 80 3/6 0/2 0/2 0/2 90 4/6 1/2 0/2 0/2 100 6/6 2/2 1/2 0/2 110 6/6 2/2 1/2 0/2 120 6/6 2/2 2/2 0/2 ______________________________________
This method would work but it was found to take 16 to 32 sterilize pulses to be equal in performance to the deep pre-injection vacuum method discussed previously.
Simply increasing the concentration of the hydrogen peroxide vapor in the 154 cubic foot chamber was also tested at various pre-injection vacuum levels. Table F contains the data for this method.
TABLE F ______________________________________ Average Hydrogen Peroxide Vapor Sterilant Penetration into 1 cm I.D. .times. 120 cm Deep Passivated Stainless Steel Deadlegs Pre-Injection Amount of Sterilant Depth of Penetration Vacuum Level Injected per pulse (cm) (percent) ______________________________________ 10 Torr 28 grams 58 48 35 grams 60 50 42 grams 75 63 56 grams 90 75 5 Torr 35 grams 60 50 2 Torr 28 grams 60 50 35 grams 73 61 42 grams 80 67 56 grams 76 63 1 Torr 56 grams 87 73 ______________________________________
The data for a 4 pulse sterilization cycle shows that increasing the concentration will enhance penetration somewhat but will not result in the desired level of penetration performance. Residual levels were higher after aeration when increased amounts of sterilant were introduced. This is presumably because the saturation, or dew point, conditions were exceeded and condensation occurred. Further increase in the amount injected resulted in excessive condensation and prolonged aeration as well as decreased depth of penetration.
The direct coupling process described in U.S. Pat. No. 4,372,916 is not always practical because all dead ended configurations must be converted to flow through configurations in order to implement such a method. This restructuring would be particularly impractical for objects contained in, for example, a freeze dryer chamber and condenser.
There is a need for a method which can sterilize complex objects by using low vapor pressure chemical vapor sterilants. There is a further need for enhancing the penetration of such sterilants into the openings and apertures of such complex objects being sterilized. There is a further need for a method which can be used in both small scale applications and large scale applications without being prohibitive with respect to cost of sterilization cycle time.